In general, communication systems utilizing wideband communication techniques convey large amounts of data via a single communication link over a wideband channel. Single-carrier wideband communication techniques typically exhibit marked signal deterioration proximate the limits of the wideband channel relative to the center (carrier) frequency. This is due in a large part to the difficulty of producing a substantially flat response over the entirety of the wideband channel bandwidth.
Multiple-carrier (multi-carrier) wideband communication techniques divide the wideband channel into multiple subchannels. Each subchannel has a signal (i.e., a modulated carrier) having a relatively narrow bandwidth. This relatively narrow bandwidth allows each subchannel to have a substantially flat frequency response. Through normalization and other techniques, these substantially flat response subchannels are concatenated to produce a substantially flat frequency response over the wideband channel. This technique is also known as multiple-tone (multitone) communication.
Efficient implementations of multi-carrier communication systems (e.g., those sampled at the Nyquist rate) allow the sub-carrier frequency response to overlap. The overlapped sub-carrier frequencies are typically required to be orthogonal. Systems utilizing such orthogonal multi-carrier techniques are often referred to as orthogonal frequency-division multiplexing (OFDM) systems.
OFDM communication techniques provide improved performance over other wideband communication techniques. Utilizing an OFDM communication technique, a wideband communication system utilizes a wideband channel frequency-multiplexed into a plurality of narrowband subchannels. Since each subchannel is not in itself wideband, circuitry may readily be devised that produces a substantially flat response over each subchannel bandwidth.
An OFDM communication technique is a discrete multitone (DMT) modulation technique by which optimized algorithms may be utilized to appropriately allocate energy and bits to each of the plurality of subchannels. This allows reliable data transfers at high rates.
OFDM techniques desirably utilize a contiguous set of subchannels, thereby providing an easily implemented wideband channel. This is typical of wireline (i.e., hard-wired) communications systems. DMT modulation has been successfully implemented for asynchronous digital subscriber line (ADSL) communications providing improved high-speed data transfers over ordinary twisted-pair lines.
Problems arise when there are breaks in the spectrum of the wideband channel, i.e., when all usable subchannels are not contiguous, as is often the case with wireless communication systems. This may occur when some subchannels contain excessive noise or other interference, or are disallowed for any reason. Several techniques have been developed to compensate for such noncontiguous spectra. Conventionally, such techniques tend to involve the use of a static assignment or non-assignment of each subchannel. That is, the OFDM signal simply does not use a contested subchannel. Static subchannels presume a static wideband channel and therefore represents a considerable inefficiency.
In wireless OFDM communication, the RF spectrum of the wideband channel is often dynamic. For example, a dynamic spectrum may be found where an OFDM communication system shares a portion of the spectrum with one or more “foreign” systems (e.g., any RF system other than the OFDM system itself). In this case, subchannels need to be dynamically assigned on a non-interference basis. This may be done in the OFDM system, in the foreign system, or both.
Dynamic spectra are often encountered with mobile communication systems. The very movements of components of the system produce ever-changing transmission paths. Such dynamic transmission paths are therefore subject to variant shadowing, noise, and interference. Even when the communication system is a fixed point-to-point system, atmospheric conditions, external mobile entities (aircraft, motor vehicles, etc.), and local noise sources (construction equipment, factories, other communication systems, etc.) may result in a dynamic spectrum.
In an OFDM communication system utilizing a wideband channel having a dynamic spectrum (i.e., a dynamic wideband channel), all or a portion of the subchannels are dynamic. That is, individual subchannels may be subject to variations in noise, interference, and transmissibility over both time and frequency. Any such dynamic subchannel may be theoretically usable, partially usable, or unusable at any given time.
Several techniques have been developed to compensate for dynamic spectra. One such technique typically incorporates scanning the wideband channel and providing transmission over only those subchannels that are clear, i.e., are fully usable. That is, that technique uses a scheme to provide selective assignment or non-assignment of each subchannel, rejecting any subchannel having more than a minimum threshold of noise, interference, and/or transmissibility. This, too, represents a considerable inefficiency.
Such a selective-assignment technique may itself be dynamic. That is, data may be transmitted in packets, with the selective-assignment scheme scanning the wideband channel and selectively re-assigning subchannels for each packet. Such a dynamic-selective assignment technique provides a marked improvement over a static selective-assignment technique, but still exhibits a pronounced inefficiency.
An OFDM communication technique is therefore needed that provides sufficient quality of service (QoS) with a high data throughput over a dynamic wideband channel. Such a technique should optimize efficiency by utilizing each subchannel within the wideband channel to its fullest.